1. Field of the Disclosure
The present invention relates to a detection circuit and a method for detecting a contactless data storage device within a communication range of an antenna. Specifically, the present invention relates to a detection circuit and a method for detecting within a communication range of an antenna for emitting an electromagnetic field a contactless data storage device which is configured for inductive coupling with the electromagnetic field.
2. Related Art
Data storage devices such as smart cards or identification tags which can be accessed without any electrical contact for reading and/or writing purpose have become increasingly popular. Particularly, for person or product identification applications radio frequency identification (RFID) systems are in widespread use. In RFID systems, contactless communication between a contactless data storage device and a reader or read/write device is based on inductive coupling of a RF field, i.e. an electromagnetic field which is emitted by an antenna of the read/write device, and a conductive object, typically a coil integrated in the data storage device. Inductive coupling occurs within the communication range of the reader device, i.e. in the vicinity of the antenna, as soon as the RF field is switched on. Accordingly, for detecting, identifying and/or authenticating a passive contactless data storage device within the communication range of the read/write device, the RF field is switched on periodically, e.g. every 250 milliseconds, a modulated identification/authentication signal with a duration of a few milliseconds is emitted, and a response from the data storage device is detected. In setups where the read/write device is not connected to electrical power lines but relies on batteries, the periodic emission of the identification/authentication signal is inefficient because it consumes too much electrical power, particularly, when no data storage device is present. Ideally, the identification/authentication signal would only be emitted, if a contactless data storage device is present within the communication range of the read/write device.
WO 03/052672 describes a contactless integrated circuit reader comprising a tuned antenna for emitting a magnetic field and means for detecting the presence of a contactless integrated circuit, comprising a coil, within a communication perimeter of the reader. According to WO 03/052672, short magnetic field pulses having a duration of 10-50 μs are emitted and by way of a data receiving circuit, which comprises a rectifying diode and a low pass filter, an envelope signal having envelope pulses corresponding to the magnetic field pulses is extracted at the antenna. The amplitude of the envelope pulses is monitored and, if at least one envelope pulse has an amplitude below a detection threshold, an object is assumed to have been introduced into the communication perimeter. Subsequently, a normal identification/authentication request signal is emitted so as to determine whether the object that has entered the communication perimeter is a contactless device or not. The method of WO 03/052672 requires a receiving circuit to generate the envelope pulses, before the presence of a contactless device can be detected.
U.S. Pat. No. 7,598,872 describes a method for detecting identification media within the communication range of an antenna for transmitting and receiving RF signals of a read/write unit which operates in accordance with the principle of inductive coupling of an RF field in the MHz frequency band, e.g. 10 MHz. According to U.S. Pat. No. 7,598,872 a short polling signal, e.g. 5 μs, which is much shorter than the normal identification/authentication signal and contains a number of fundamental (carrier) oscillations of the RF field, is periodically emitted via the antenna. In order to avoid the transient phase of amplitude build-up of the carrier oscillation at the beginning of the polling signal, a return signal with a number of stable fundamental oscillations of the RF field is detected at the antenna at a defined time delay, after the beginning of the emission of the polling signal, for example, in the second half of the polling signal, e.g. after a time delay of 3 μs. Subsequently, the detected return signal is compared to a reference signal, and if the return signal differs from the reference signal, an identification medium is assumed to be within the communication range of the antenna, and a normal identification/authentication request signal is emitted for detecting, identifying and/or authenticating the identification medium. Within the delay time, settling processes can take place so that in the detected measurement range the detected return signal includes only stable fundamental oscillations. On one hand, limiting the return signal to stable oscillations requires a time delay in the detection process; while on the other hand, the return signal can be compared to the reference signal, before the end of the polling signal.